Proof of the carcinogenic effect of asbestos has been available since the end of the 1950's. Further research in recent times has shown that the carcinogenicity is not confined to asbestos but that any fibrous dust in the form of elongated particles is liable in principle to initiate carcinogenic mechanisms which according to the present state of scientific knowledge differ distinctly from the carcinogenic mechanisms of other chemical substances or ionizing radiation.
For reasons of industrial health and safety in dealing with fibrous dust, there has been in existence since the 1960's a scientifically recognized definition of an inhalable fibre which may be toxicologically active. This definition relates to a fibre recognizable under the optical microscope as having a geometrical diameter of &lt;3 .mu.m, a length of &gt;5 .mu.m and a length/diameter ratio of more than 3:1. This definition is based on the findings relating to the carcinogenic effect of asbestos,
Man made mineral fibres such as glass fibres, basalt fibres, slag fibres and ceramic fibres which are produced in some cases in the form of short fibres may also come under this definition. For many technical applications it is preferred to use man made mineral fibres (KMF=kunstliche Mineralfasern) having a much smaller geometrical diameter than 3 .mu.m. e.g. so-called micro glass fibres made of C- and E-glasses, which have fibre diameters of from 0.1 .mu.m to 5 .mu.m. Man made mineral fibres which have been produced by known processes for insulating purposes, e.g. by centrifugal basket processes, centrifugal wheel processes or blow processes, also contain a proportion of fibres having a diameter of less than 3 .mu.m and to some extent even less than 1 .mu.m. The fibres mentioned here have been described, e.g. in [1]. Such fibres are of great technical and economical interest for a wide variety of uses.
In places in which man made mineral fibres are produced or used, the fibres in the atmosphere may on average be shorter and thinner than those in the finished products. Transport mechanisms and forms of distribution and transformation of man made mineral fibres dusts are illustrated in the literature reference [2] which also gives data concerning exposure to fibres entering the lungs in the production and processing of man made mineral fibres. Further information may also be obtained from [3]. Detailed information on the carcinogenic effect of man made mineral fibres is now available as a result of numerous scientific investigations carried out worldwide. These include inter alia animal experiments such as inhalation tests, intratracheal and intraperitoneal experiments and cytobiological and other in vitro studies. A summary of results is given in [4] in which the carcinogenic effect is determined by the cooperation of the following two factors:
1. The fibrous form, e.g. in accordance with the above definition and PA0 2. the persistence (length of stay) in the lung. PA0 1. the so-called translocation, e.g. by macrophages, and PA0 2. the solubility of the fibres.
The most conclusive results of comparisons between natural and man-made mineral fibres are obtained from animal experiments in which the dusts are administered intraperitoneally or intrapleurally as these are able to produce spontaneous rumours. According to [5] and [6], IARC has undertaken to subdivide man-made mineral fibres into carcinogenic and non-carcinogenic fibres. According to this classification, man made mineral fibres such as thin glass fibres, stone fibres and ceramic fibres as well as many types of asbestos may be carcinogenic. Thick glass fibres and unstable glass fibres are non-carcinogenic. The stability of man-made fibres depends to a large extent on their chemical composition. The length of stay of fibrous dusts in the lung (persistence) depends on the composition and size of the dust particles.
The greater the chemical stability and the greater the geometrical diameter of the fibres, the greater will be the persistence.
Reference [4] gives results of more recent intraperitoneal experiments which unequivocally prove the carcinogenic effect of various man-made mineral fibres such as basalt fibres and special micro glass fibres. It is surprisingly found that glass fibres having an average fibre diameter of very much less than 1 .mu.m are also highly carcinogenic. It is known that such fibres have a high chemical stability due to the composition of the glass. Important guidelines for the solubility of man-made mineral fibres in vivo and in vitro are found in [7]. The significance of the chemical composition for carcinogenesis is investigated in [8], in which it is found that fibres which have been pretreated intensively with an acid have no more tumour producing effect than untreated fibres.
There is scientific evidence that the carcinogenic effect of man-made mineral fibres depends to a high degree on the capacity for removal of the fibres from the lungs. This capacity will hereinafter be referred to as "lung clearance". It is determined by animal experiments and depends on two factors, namely
In inhalation experiments, there is possibly the added factor of alveolarclearance. Clearance experiments in rat lungs after intratracheal installation of fibres are described in reference [9] which also gives data on the half life times of lung clearance for various mineral fibres, in particular glass fibres.